Rejection of line interference



c. M BYERLY 2,758,299

REJECTION oF LINE: INTERFERENCE 5 Sheets-Sheet 1 Aug. 7, 1956 Filed NOV.17, 1950 Aug. 7, 1956 c. M. BYERLY REJECTION oF MNE: INTERFERENCE 3sheets-sheet 2 Filed Nov. 17, 1950 C. M. BYERLY REJECTION OF LINEINTERFERENCE Aug. 7, 1956 5 Sheets-Sheet 5 Filed Nov. 17, 1950 UnitedStates Patent REJECTIoN oF LINE INTERFERENCE Coy M. Byerly, Culver City,Calif., assignor to Gilfillan Bros., Inc., Los Angeles, Calif., acorporation of California Application November 17, 1950, Serial No.196,310

A1 Claim. (Cl. 343-5) The present invention relates to improved meansand technique whereby video signals may be transferred to remotelylocated stations without substantial interference due to electrostaticor electromagnetic influences exerted by neighboring lines carryingcurrents of low frequency; for example, 60 cycles; and, specifically,the present invention relates to improved means and technique wherebythe video and synchronizing signals in a radar system may be transmittedto a remote location over the same transmission line while preservingthe fidelity of the video.

The present invention contemplates an improvement in a ground controlledapproach (G. C. A.) radar system in which the echo receiving antennamay, for example, be located adjacent an aircraft landing strip with theassociated indicator located at a remote position, as for example, in acontrol tower which may be located as much as two miles from the antennaitself.

In such systems a part of the echo receiving system is located adjacentthe antenna in which the received echoes are detected, and the resultingvideo is transferred over a transmission line, such as a coaxial cable,to the control tower at which the video is applied to a cathode ray tubeto produce visual indications of the video.

Such video is comprised of different frequency components extending, forexample, from 60 cycles per second to ten megacycles, and it isdesirable that the video be conveyed without appreciable loss infidelity from the antenna to the remotely located cathode ray tubeindicator. One of the chief factors contributing to loss in fidelity insystems of this character results from interfering voltages induced intothe connecting lines from neighboring power lines. These voltages,induced either electromagnetically or electrostatically, are, of course,of power frequency and have caused considerable trouble in the past,particularly since these pickup voltages may not remain constant eitherin amplitude or in phase Heretofore different attempts have been made toovercome the deleterious effects produced by such interfering voltagessuch as, for example, using a shielded twisted cable pair and low passfiltering networks. Such networks leave much to be desired since theyrequire elaborate design, are bulky, and themselves introduceundesirable time delays, and require adjustment, depending upon theintensity of the voltage induced and its phase. This problem is mademore difficult in those instances where the level of these interferencevoltages exceeds that of the transmitted video signal.

lt is therefore an object of the present invention to provide improvedmeans and technique whereby the effect of the interfering voltages ofpower frequency is minimized, whereby the video may be transferred to aremotely located station without appreciable loss in fidelity.

Specifically, an object of the present invention-is to provide means anda teaching whereby the effect of interfering signals of power frequencyinduced in a long transmission line carrying video frequencies may beminimized.

Another specific object of the present invention is to provide animproved radar system in which both the synchronizing signals and videoare transferred over the same transmission line extending to a remotelylocated indicating station without deleterious effects produced byinduction of voltages of power frequency from neighboring power lines.

Another specific object of the present invention is to provide meanswhereby the low frequency components of the video signal are selectivelyamplified, or preemphasized, in relationship to the high frequencycornponents thereof, before application to the long transmission line,and in such relationship to the expectant induced voltages of powerfrequencies from neighboring power lines, to minimize the effect of theinduced voltages. Q

A further specific object of the present invention is to provideimproved means and technique whereby the low frequency components of thevideo signal are selectively amplified or preemphasized beforeapplication to the transmission line, and then subsequentlydeernphasized at the remotely located receiving station, to minimize theeffects of induced voltages of power frequency from neighboring powersources or lines.

The features of the present invention which are believed to be novel areset forth with particularity in the appended i claim. This inventionitself, both as to its organization and manner of operation, togetherwith further objects and advantages thereof, may be best understood byreference to the following description taken in connection with theaccompanying drawings in which:

Figure 1 shows in schematic form a portion of a radar system embodyingfeatures of the present invention;

Figure 2 is a circuit diagram showing a portion of the apparatusincluded in the Transmitter remoting circuit with low frequencypreemphasis element shown in Figure l;

Figure 3 is a circuit diagram showing the circuit cornponents in theReceiver'remoting circuit with low frequency inverse deemphasis shown inFigure 1; and

Figure 4 is a graph showing the relationship of gain to frequency in thetransmitter remoting circuit shown in Figure 1 and is helpful indescribing the operation of Figure 2.

Although the present invention in its broader aspects has generalutility where it is desired to transmit video or other intelligence overan extended length of transmission line which is subjected to botheiectromagnetically and electrostatically induced voltages, the presentinvention is described as being incorporated in a radar system shown inFigure l.

In Figure 1, the central timer or synchronizer 19 generates pulses ortrigger voltages, referred to hereinafter as triggers. These triggersare delivered to the modulator 11, which causes the transmitter 12 todeliver high frequency energy to the antenna system 13 in timedrelat'ionship with such triggers. The resulting echoes from reflectingbodies in the path of the electromagnetic beam radiated from the antennasystem 13 are received in the receiver 14, which is coupled to theantenna system 13 through the transmit-receive (TR) switch 15. Thereceiver 14 is of the conventional superheterodyne type and the outputof the second detector stage constitutes video represented at 16. Thisvideo 16 contains the peaks 16A, 16B, which correspond to the receivedechoes.

It is desired to transmit this video 16, as well as the synchronizingpulses or triggers 17, to the cathode ray tube indicator 18 to obtainvisual representations at a remote location; for example, in the controltower of an airport, with the other components of the radar systempreviously described located adjacent the aircraft landing strip. Thesetwo locations, i. e., the landing strip and control tower, may beseparated at a distance of, for example, two miles, and the video 16 andsynchronizing pulses f17 are't-ransferred'between such stations over thecentral inner .conductor of asingle coaxial transmission line 19 whichhas its outer sheath grounded. For this purpose, the video 16 andsynchronizing triggers 17 are applied to thetransmitter-remoting-circuit'with -low nfrcquency `preemphas'is 21 inFigure I1, this -circuit `being shown :in more detail -in -Figure 2.

The remote terminals of the vvtransmission 'line 19 are connected to thereceiver remoting circuit with `low'frequency deemphasis 22, -whichisshown-infdetail gin Figure 3. One'ofthe-output terminals -2-2A oftheremoting circuit 22 is connected -zto Vthe --input--terminal ofthe videoYamplifier '23, the -output of which --is connected, in turn, 'to thecontrol grid `25 ofthe cathode -ray tube 181e effect intensification `ofAthe -cathode ray -beam -in accordance with .the echo pulses 116A, 16B.'Thesotheroutput terminal .22B of-the iremoting circuit 22 \isconnected-to laninpnt terminal of the sweep amplifier and sweep limiterstage 27, to cause cathode raybeam deflecting curren-ts'to flowperiodically, in timed relationship with tthe pulses 17, through vthequadraturely spacedde'flecting -coils 2-8, 29 associated with thetube18. Oneofthe-terniinals-of lthe sweep .amplifierand -sweep limiter stage27 `-is connected to-the blanking voltage generator vstage 30, havingits loutput applied .to thecathode of -the tube '-18 -to cause thecathode ray beam to become invisible lat -predetermined times.

The radar-system thus .far ydescribedlis essentially conventional, ,withthe exception of the terminal circuits '21, 22, and foramore detaileddescriptionof the-components shown in block diagram herein, referencelis had to the copending patent applicationof Tasker-et al., Serial No.776,702, filed-September 29, '1947,

Referring to .Figure 2, the remotingcircuit 21 has the input terminal21A to whichv the video 16 is-applied. This video includes the echopulses 16A, 16B, each-having a duration of approximately 0.5microsecond, Yand ithepre- Cedingpulse 31 .in 4the video .wave 16,-which is not an echo pulse, is due to imperfection in the TRswitch'115.

The echo pulses .of ;.the ;vi deo 21'6 comprise, as determined byFourier analysis, frequency components with appreciable amplitudeextending, for example, from .-60 cycles per-second to '10;megacycles.The:firstandsecond amplifier stages 32, 33 .in ;Figure 2 serve -itoselectively' amplify or -preemphasize :the :low lfrequency components ofthe video with respect to the high frequency\components, totobtain Vthegeneral results indicated =in iFigure 4. For this purpo se;lowfrequencyfpreemphasis :networks 34, 35 are connectedlin:the-outputcircuits of the Stages 3.2, 33- The video signal-afterrthus being selectively amplified in stages- 32, 33 -isapplied.to'zthe-control grids of the parallel connected cathode:follower stages 37, 38, the cathodes of which are each connected to thecentral conductor of coaxialcable 19. `Onthe `other hand, the trigger`17 is appliedto'theinput 4t err ninal..-21B.of:the remoting circuit 21.This terminal 21B. -,Serves.as aninput terminal for the triggeramplifier and blocking oscillator stage 39, the output terminali-of.which is connectedrto the inner conductor-of thecoaxialcablc 1.9.The-:Stage 39 is for the purpose of conditioning the synchronizing-pulseof trigger 17 for transmission -over -the dine v1.9.,.andpernploysconventional circuitry for amplifying and peaking, i. e., renderingsharper, the synchronizing vpulses 17.

Fora moredetailed description of VFigure 2, itis noted that the-tube 32is apentode andmayxbeia 6AG7 with'the control grid 40connectedthrough.condenser 41 --to wthe variable tap on thepotentiometerftype resista-nce 42, one outsiderterminal of which-isgrounded and the other outside terminal of which isconnectedtothe input:terminal 21A. input resistance .4 3 is connected in shunt with thepotentiometer lresistance 42. -Bias for-.the--control grid 40 issupplied from the 150.volt1source 4 6, which 'has one of its terminals`grounded and the other tone v.0f its terminals connected-through1resistance 47, 4 8 to the igt-id 40; the junction point of.resistance.47,48 beinggrouncled through resistance 49.

terminal -connected to the -gridy 66.

4 Space current for the device 32 is supplied from the ung-roundedpositive terminal fofvoltage source 52, `which is connected throughresistances 53 and 54 to the anode 55. The cathode 56 is grounded. Thescreen grid 58 is bypassed to ground through bypass condenser 59 and isconnected through resistances 61 and 53 to the positive terminal ofsource 52. lhejunction point of resistances 53 and 54 is connectedthrough condenser 63 to ground, so that-theresistances 53'and 54 andcondenser v63 effect a larger .amplification :of Vthe 10W .frequencycomponents of video signal than the high frequency/components, asindicated in the curve 64 (Figure 4). For this purpose the resistance 53maybe 1,0 00 ohms, .the resistance 54 may be'330 ohms, andthe condensermay"be'0.5-microfarad.

The frequency components thus selectively amplified in stage 32 andappearing at the anode 55 are applied through coupling condenser 65 tothe control grid 66 of stage '33, which -may comprise an 807 type'tu'bevThe cathode G7-is grounded, land bias -for `the 'grid 66 'is obtainedagainst the iresistance 68 having Y'its ungrounded The lanode 69 yisconnected, 4fortheilow of yspace current, to 'thepositive -terrninal-ofsource `52m-rough tthe series circuit comprisving a small resistance7-0, resistance 71 rand resistance 72. The-screen ygrid '74 Ais bypassedto vground through the bypass condenser 75, and connected through -resistances '-7-6, 77 and 72 to'the'positiveterrninal-of source '52,The-junction point-of resistances'71 land 72 ris connected Ato groundthrough the-condenser-SO. `Resistances 7l-and 72may each be '50() ohms,condenser `80 may be i 0.5 microfara'ch :and together Aconstitute -a'low frequency preemphasis -circuit functioning in the same manner asthe previously -described low frequency 'preemphasis circuit 34 toselectively-amplify -the low-frequency compo Vnents of fthe Avideo `'toobtain 'the result 'indicated -by the curve 581 in YFigure f4.

-In 'Figure 4 the overall effect -of the stages '32, 33,represented-respectively by the curves :64 'and l8-1, is in--dicatediiucurve '82. The-abscissae of the curves 64,81

and 82 each represent frequency and extend over a 4raugeof-'tSO-cyclesto l0 megacycles, lwhile-the ordinates of -thesetcurves represent a-ratio of lgain at'the particula-r'frequency-concerned to the; gainat-the frequency midlIt lis thus "The frequency components thusIselectively amplified Vare applied through coupling condenser 84,through re- -fsistances 85, A86, `to -the control grids 87, 88 of theparallel connected cathode follower 4stages 37, '38. -is noted that-'one.terminal -of coupling condenser -84 ris connected to the I'junctionApoint ofresistances 70 and 71.

'The cathodes lrif-'stages v37, 38-are each connected to the centralconductor of --the coaxial Icable 19 and y'are tive ungrounded terminal4'of source 46 through the `serially connected =resistances :92 and r93,the junction point-of -wli'ich is grounded through resistance `.94. Agermanium crystal '95 is connected inshunt withV resistance 93 withpolarity indicated, rand serves -as a clamping ydiode toypreventfbloclting of the stages 37, v38, as `a resultof high positive'voltages applied -to the grids.

The output of 'the trigger amplifier stage 39 .is 'likewise applied tothe nngrounded terminal of theloadresistance 90. Thus., acompositesignal-of video and synchronizing pulses is applied .to theinner ,conductor 4of cable 19 `for transmission. .to remotely .locatedequipment, shownin Referring ,to -Figure 3, 'the composite vvideo ,andtrigger :signal 10!) appearing on :the inner conductor of the coaxialcable 19 is applied through Condenser 101, on the one hand, to thecathode 102 of the grounded grid amplifier 103, and, on the other hand,to the input terminal 104 of the Trigger amplifier and blockingoscillator stage 105. The signal 100 thus applied to the cathode 102differs from that at the transmitting circuit 21, in that the amplitudesof all components may be smaller, and attenuation and phase shift in thehigher frequency components is greater in relationship to thecorresponding attenuation and phase shift appearing in the lowerfrequency components, In general, the equalizer network 107 introduces acompensatory etect for the attenuation and phase shift of the componentsof the signal resulting from the characteristics of the coaxial line 19.Also, extraenous signals induced by electrostatic and/ or electro`magnetic coupling between the elements of the coaxial line 19, and, forexample, nearby power lines, form a component of the video signal thusapplied to the cathode 102. These induced voltages, at powerfrequencies, may have an amplitude comparable to that of the echosignals applied at the input terminal to the coaxial cable 19. Thesignal amplied in tube 103 and appearing at anode 109 is equalized byequalizer stage 107 and applied to the low frequency deemphasis network110, and more specifically to the control grid 111 of tube 112 throughthe coupling condenser 114.

The low freqency deemphasis circuit includes generally two components,namely, the circuit, i. e., the input circuit connected at the cabletermination which consists of the condenser 101, resistances 138 and139, and input resistance of the grounded grid amplifier tube 103; andsecond, the low frequency deemphasis circuit 110. The condenser 101 hasa magnitude of two microfarads, and the combined effective resistancevalue of the resistances 138, 139 and tube 103 is preferably equal tothe magnitude of the characteristic impedance of the cable, 75 ohms inthis instance. The low frequency deemphasis circuit includes thecoupling condenser 114 of, for example, 3300 micromicrofarads,resistance 115 of, for example, 10,000 ohms, a potentiometer resistance116 of, for example, 250,000 ohms and a resistance 117 of, for example,1,000 ohms. The condenser 114 is connected between the anode 109 and thecontrol grid 111, and the resistance 115 has one of its terminalsconnected to the control grid 111 and the other one of its terminalsconnected to an outside terminal of potentiometer resistance 116. Theother outside terminal of potentiometer resistance 116 is connected tothe ungrounded terminal of resistance 117, which is likewise connectedto the variable tap on potentiometer resistance 116.

In general, adjustment of the resistance 139 results in adjustment ofthe value of the termination resistance to produce compensatory elTectsfor different tubes which may be used to replace the tube 103. Thisresistance 139, for a given tube, is adjusted so that the effectiveresistance of the elements 138, 139 and 103 is 75 ohms, as mentionedabove. By thus terminating the cable 19 to a resistance network whichhas substantially the same resistance as the surge impedance of thecable 19, through the condenser 101, some deemphasis is obtained,particularly when, as in this instance, the condenser 1.01 has amagnitude of 2 microfarads. The final adjustment, in the nature of aVernier adjustment, is obtained in adjusting the elements of the lowfrequency deemphasis network 110.

In the particular installation, satisfactory results are obtained whenthe deemphasis circuit described above attenuates the 60 cycle signalsmore than the preemphasis circuit in the transmitting circuit 21 booststhat particular frequency. However, this adjustment is not critical andsatisfactory results may be obtained over a wide range of adjustment.

The high and low frequency components thus selectively applied to thecontrol grid 111 are amplified in the wide band video amplifier stages120, 121, which include respectively the discharge devices 112 and 123.The signal after amplification in device 123 is applied through couplingcondenser 125 to the control grid 126 of the cathode follower stage 127,and Lthe resulting video appearing across the cathode resistance 128 isapplied, as shown in Figure 1, to a video amplifier stage 23 for furtheramplication to the control grid 25 to elfect relative changes inintensity of the cathode ray beam.

On the one hand, `the output of the Trigger amplifier and blockingoscillator stage is applied to the stage 27 in Figure 1 for achievingthe above purposes.

Thus, the transmitted video signals having the general appearance shownat 100 in Figure 3 are transformed into the corresponding compositesignal represented at 125. This signal represented at appears across thecathode resistance 128, whereas the signal represented at 100 appears onthe cathode 102.

For a more detailed description of Figure 3, it is noted that theequalizer stage 107 has one of its terminals connected to the anode 109and the `other one of its terminals connected through resistance 130 tothe positive ungrounded terminal of the Avoltage source 131. The network107 includes four shunt connected arms, the iirst arm including thecondenser 132, the second arm including the condenser 133, the third armcomprising the resistance 1'34, and the fourth arm including aninductance coil and variable resistance 1236. The cathode 102 isreturned to ground through the seriall f connected resistance 138 andvariable resistance 139, to thereby provide a path for the ilow of spacecurrent through device 103 and to normally maintain the cathode 102slightly more positive than the associated ground grid 140.

The screen electrode 141 of device 103 is bypassed to ground through thebypass condenser 142 and supplied with space current through theserially connected resistances 143, 144 and 130. A bypass condenser 145has its ungrounded terminal connected to the junction point ofresistance 130 and 141.

The cathode of the pentode 112 is grounded and the anode of device 112is connected to the positive terminal of source 131 through the seriallyconnected circuit which includes the resistance 148, peaking coil 149and resistance 150, the junction point of coil 1149 and resistance 150being connected to the ungrounded terminal of condenser 152. Likewise,the junction point of coil 149 and resistance 150 is connected to thescreen electrode of device 112 through the serially connectedresistances 154 and 155, such electrode being connected to theungrounded terminal of bypass condenser 156. It is noted that the wideband amplifier stage 121, which includes the tube 123, is substantiallythe same as the wide band amplier stage 120, which includes Ithe tube112, and for that reason a detailed description of the manner in whichthe circuit components of stage 121 are interconnected is believedunnecessary.

The ampliiied signal thus appearing on `the anode of device 112 isapplied through coupling condenser 150 through the control grid ofdevice 123, the anode of which is coupled .through condenser 125 to thecontrol grid 126 of the cathode follower stage. The control grid 126 isreturned to ground through the serially connected resistances 162 and163, the junction point of which is connected to the negative ungroundedterminal of source 165 through lthe resistance 166. Also, a clampingdiode in the form of a germanium crystal 167 may be connected in shuntwith the grid resistance 162. Thus, the control grid 126 is maintainedslightly negative with respect to its cathode. Source 165 likewiseprovides a bias potential for the control grid 111 of the wide bandampliier 112. In this respect it is noted that the junction point ofresistances 116 and 117 is connected through resistance 168 to theungrounded negative terminal of source 165, whereas the main controlgrid vention have abeen fshown and described, it will lbe obvious to.those skilled =in the art that vchanges vand modixcsations may befmadewithout departingtfr'omthis invention in its broader aspects and,therefore, the yaim in the appended claim Lis to -eover all such changesand modifications as fall within the true spirit and scope vof thisinvention.

-I.clain1:

In a radarsystemofsthe characterdescribed, wherein it .is desired ltottransmit both videofandsynchronzing signals from asending station to aremotereeeiving-s-ta- ,tion over a transmission line which is `subjectAto interference lfrom neighboring low frequentwtpower lines or sourcesvWithout `substantial ,interference due to neighboring power circuits,asource=of;triggers,;rst means s ensitive t-o said :triggers for@deriving .video echo ysignals Ain timed relationship with saidtriggers, a transmission line .substantially free lfrom currents -ofpower -frequencies extending Ifrom `said sending station Ato said remote'receiving station, -second rmeans coupling Vsaid lsource oftriggers-'to said transmissionline at said sending station, a sourceofsaid video echo signals derived I in timed lrelationship with saidtriggers, low frequency preernphasis, means connected between saidsource and said transmission lline at :said receiving station, said4second 4coupling means :bein-g effective to couple saidsourceofttriggers to said line without said triggersbeing iniluenced b,yl said vpre-emphasis means, Avideo vreproducing means, lowfrequeny-deemphasis meansiconnected -betweensaid `video reproducingmeans and said transmission .line at said receiving station, vand thirdmeans coupled :between said ivideo :reproducing means and saidtransmission line at said receiving station -for operatinf,Y said yvideoVreproducing s means in -ltimer relationship with Isaid triggers -saidthird means :beingfeffective t-o couple said triggers t-o'saidvideo'reproducing means withoutsaid triggersfbeing i-nuenced by said.de-eniphasis imcans.

References .Ctedzin the le-of thispatent vUNITED STATES :PATENTS TinusNov. 4, `1952

